A secondary tremor measuring between 4.1 and 4.6 on the Richter scale was detected around eight-and-a-half minutes after North Korea's latest nuclear test around noon on Sunday.
But what caused it? Experts believe it was caused by the collapse of the ground layer surrounding the test site in Pungye-ri, North Hamgyong Province. The phenomenon was not detected during the North's last five nuclear tests.
China's meteorological service said the quake measured 4.6 on and attributed it to a "collapse." The U.S. Geological Survey said it measured 4.1 and blamed a "structural collapse" related to the initial blast.
But the Korea Meteorological Administration detected nothing. "It appears that the secondary tremor was detected only in China, which was closer to the nuclear test site," a KMA spokesman said. "The meteorological observatory run jointly by South Korea and China is 350 km away from the test site and did not pick it up."
The spokesman added an earthquake of that intensity is seen when a large mine collapses.
According to the KMA, the latest nuclear test happened around 2.2 km under the surface. The collapse is believed to have resulted from either the intensity of the latest blast or due to ground layers being weakened by previous nuclear tests.
Seo Kyun-yeol of Seoul National University said, "A basin could have formed after some of the ground layers collapsed. The collapse could also have caused fractures in the underlying layer, raising concerns of contaminating groundwater and other layers of earth."
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